This invention relates to a method of making thermally durable glass-ceramic articles having strongly adherent coatings thereon.
The term "glass-ceramic" refers to a polycrystalline ceramic prepared by the controlled crystalization of a glass in situ. The invention of such materials has enabled the formation of intricately-shaped polycrystalline articles by forming "green glass" into the desired shape and thereafter ceramming the green glass article to convert it to a glass-ceramic. The ceramming step conventionally involves heating the green glass article to an elevated temperature, e.g. a temperature in the range of about 650.degree.-800.degree. C., to cause nucleation or the formation of nuclei and subsequently heating the article to a higher temperature, e.g. a temperature in the range of about 800.degree.-1175.degree. C. to cause crystallization and growth of crystals. The resultant glass-ceramic material is known for its good mechanical and thermal durability. For additional information pertaining to the formation of glass-ceramic materials, reference may be made to U.S. Pat. Nos. 2,920,971; 3,148,994 and 3,157,522. For example, U.S. Pat. No. 3,157,522 indicates that the nucleation step can be performed immediately after the article is formed while the article is still hot. Thus, the shaped green glass article may be cooled to a temperature in the range of 650.degree.-800.degree. C. and held at that temperature for a period of time, usually between about 2 hours and about 10 minutes, depending upon the temperature. This type of nucleation step may be performed in the method of the present invention when the article is to be abraded while in the nucleated state. Furthermore, the abraded article may be subjected to two or more crystal growth temperature schedules in accordance with the aforementioned U.S. Pat. No. 3,148,994. However, it is to be noted that the method of the present invention is not limited to any particular ceramming schedule, and any ceramming schedule may be employed since all such schedules are capable of alleviating damage caused by a prior, surface abrasion step.
Glass-ceramic articles have been coated with electrically and/or thermally conductive material to form such devices as resistors, heaters, stovetop cooking units, cookware and the like. The following references are illustrative of these applications: U.S. Pat. Nos. 3,330,940; 3,813,520; 3,848,111 and 3,883,719 and my U.S. Patent Application Ser. No. 727,893, entitled "Low TCR Resistor" filed on Sept. 29, 1976. Any of the articles disclosed in the aforementioned references may be subjected to temperature changes which can cause the formation of stress therein which can result in breakage of the article and/or separation of the conductive coating from the glass-ceramic substrate.
Although glass-ceramic materials have been commonly employed as cookware, a primary disadvantage of such ware is the low thermal conductivity thereof. For example, direct contact of glass-ceramic cookware with burner elements is disadvantageous in that food disposed therein can be burned if it is immediately above the burner element, and food which is only a short distance away, but not directly over a burner element, may be undercooked.
The application of coatings to glasses and glass-ceramics is widely employed to impart desirable physical properties such as thermal and electrical conductivity thereto, and many processes for applying such coatings are known. U.S. Pat. Nos. 3,523,013; 3,220,870; 3,296,012; 3,914,517 and 3,741,780 disclose methods of providing glass-ceramic substrates with conductive coatings.
The principal problems in the art of metal-coating glass-ceramic cookware arise out of the substantial differences in thermal expansion behavior between the conventional glass-ceramic materials used for the fabrication of cookware and more highly conductive materials which might be considered for use as coatings in combination with these glass-ceramics. For example, glass-ceramic materials typically employed for cookware fabrication exhibit rather low coefficients of thermal expansion, e.g., on the order of 10-25.times.10.sup.-7 /.degree.C., whereas aluminum, for example, which has a desirable thermal conduction capability, has a coefficient of thermal expansion of about 230.times.10.sup.-7 /.degree.C. Theoretical stresses which may arise as a result of this expansion mismatch over the typical temperature range of use of an aluminum metal-glass-ceramic composite cooking vessel approach 700 MPa. Increases in the thermal expansion of the glass-ceramic material to alleviate this expansion mismatch are not possible without sacrificing the excellent thermal shock resistance of this material, a major desirable feature of glass-ceramic cookware. Even silicon, which has a coefficient of thermal expansion of about 35.times.10.sup.-7 /.degree.C., is subjected to stresses which tend to cause delamination to occur when silicon-coated cookware is subjected to thermal cycles normally encountered in cooking. Moreover, such glass-ceramic ware possesses an extremely smooth surface that is not normally receptive to many coating materials. One example of a material that can be self-bonding to smooth glass-ceramic surfaces is aluminum, provided that it is properly applied, an example of such a self-bonding material being described in U.S. Patent Application Ser. No. 712,479 entitled "Process for Making Aluminum-Coated Glass-Ceramic Cooking Vessel" filed Aug. 9, 1976.
Three techniques conventionally employed to improve adhesion of non-bonding coatings are surface roughening, substrate preheating, and precoating the substrate with a material which is self-bonding and which is compatible with the desired coating material. However, each of these techniques possesses disadvantages which tend to discourage its utilization. For example, subjecting glass-ceramic articles to high preheat temperatures was found to effect coating bonds of marginal strength, but all but the thinnest coatings spalled as the coated articles cooled. Vacuum deposited films of chromium having thicknesses of about 500 A formed good bonding layers between glass-ceramic substrates and silicon coatings, but the vacuum deposition technique for applying such films is unduly expensive. It is knwon that although surface roughening of glass-ceramic substrates improves the adhesion of subsequently applied thermally conductive coatings, such roughening introduces undesirable stress raisers and degrades the thermal durability properties of the glass-ceramic. For example, glass-ceramic skillets, the bottom surfaces of which were roughened prior to applying thermally conductive coatings thereto, could not pass a thermal downshock test because of the damage imparted thereto by the roughening procedure.